تسريب كلمة المرور والتنبيه الأمني سيئان.

Because this is a nominal sentence in the present tense. In Modern Standard Arabic, the present-tense copula is/are is usually not written or said.

So:

تسريب كلمة المرور والتنبيه الأمني سيئان
literally works like: The password leak and the security alert bad
but in natural English: The password leak and the security alert are bad.

If you wanted the past, Arabic would normally use a verb such as كان.

Because the sentence is talking about exactly two things:

• تسريب كلمة المرور = the password leak
• التنبيه الأمني = the security alert

These two noun phrases are joined by و (and), so the predicate refers to both of them together. In standard Arabic, when exactly two things are being described, the adjective/predicate can appear in the dual:

• singular: سيئ = bad
• dual: سيئان = bad (for two)
• plural: سيئون = bad (for more than two, masculine sound plural)

So سيئان means both are bad.

Because the word is in the nominative case.

In a basic nominal sentence like this one, the predicate is nominative. For the dual, the endings are:

• ـان in the nominative
• ـين in the accusative and genitive

So here the correct form is سيئان.

If the grammar of the sentence changed, you could get سيئين instead. For example, after certain particles or verbs that require the accusative, the form would change.

Because the predicate does not agree with كلمة by itself. It agrees with the main nouns being described.

The two things being called bad are:

• تسريب = leak / leaking
• التنبيه = alert

Both تسريب and التنبيه are grammatically masculine, so the predicate is masculine dual: سيئان.

The noun كلمة is inside the expression كلمة المرور and is not the head noun of the whole first subject. The head of that first subject is تسريب.

Because تسريب كلمة المرور is an iḍāfa construction (a genitive/possessive construction).

In an iḍāfa:

• the first noun usually does not take الـ
• the whole phrase can still be definite if the second part is definite

So:

• تسريب = first term of the iḍāfa
• كلمة المرور = second term

Since كلمة المرور is definite, the whole phrase تسريب كلمة المرور is definite too.

This is why Arabic does not say التسريب كلمة المرور here.

No. تسريب here is a noun, specifically a verbal noun (also called a maṣdar).

It comes from the verb سرّب = to leak / to cause to leak / to disclose.
So تسريب means something like:

• leaking
• leakage
• disclosure
• leak

Arabic often uses verbal nouns where English might use either a noun or an -ing form.

Because الأمني is an adjective modifying التنبيه.

In Arabic, adjectives usually:

• come after the noun
• agree with the noun in definiteness
• agree in gender
• agree in number
• agree in case

So:

• تنبيه أمني = a security alert
• التنبيه الأمني = the security alert

Both words have الـ because the noun is definite, and the adjective must match it.

Because the sentence is structured like this:

[تسريب كلمة المرور] و [التنبيه الأمني] سيئان

So there are two coordinated noun phrases:

1. تسريب كلمة المرور
2. التنبيه الأمني

Then سيئان describes both of them together.

If التنبيه الأمني were somehow part of the first phrase, the structure would look different. Here, the conjunction و clearly introduces a second subject.

It is another iḍāfa construction:

• كلمة = word
• المرور = passage / passing / access

So literally it is something like word of access/passage, which is the standard Arabic expression for password.

That means the sentence contains a nested iḍāfa:

• كلمة المرور = password
• تسريب كلمة المرور = password leak / leaking of the password

This kind of stacking is very common in Arabic.

With full vowel endings, it would be:

تسريبُ كلمةِ المرورِ والتنبيهُ الأمنيُّ سيِّئانِ

This shows the grammar more clearly:

• تسريبُ: nominative, because it is part of the subject
• كلمةِ: genitive, because it is in iḍāfa after تسريب
• المرورِ: genitive, because it is the second part of كلمة المرور
• التنبيهُ: nominative, as the second coordinated subject
• الأمنيُّ: nominative adjective matching التنبيه
• سيِّئانِ: nominative dual predicate

In normal Arabic writing, these short vowels are usually omitted, which is why learners often notice only the dual ending ـان.